British Patent Specification No. 1,542,252 contains a discussion of the problems associated with the well known bimetallic actuator comprising a disc of domed configuration which moves to an oppositely domed configuration with a snap action with changes in temperature, such problems including drift of the operating temperature at which the disc snaps with age, a tendency for stress cracking to occur in the disc, and the very small range of useful movement associated with the snap action.
British Patent Specification No. 1,542,252 further discusses the also well known form of actuator described, for example, in British Patent Specification No. 657,434, which comprises a rectangular sheet or blade of bimetal having a central tongue released from between two outer legs whose ends, adjacent the free end of the tongue, are joined by a bridge portion, and wherein the blade has imparted thereto a dished configuration such that, with changes in temperature, the blade moves between oppositely dished configurations with a snap action. As described in British Patent Specification No. 657,434, the dished configuration was in the past imparted to the blade by virtue of a crimp being formed in the aforementioned bridge portion, but more recently the requisite dished configuration has been obtained by pressing the blade between two dies. The actuator of British Patent Specification No. 657,434 provides a much larger range of useful movement than the known disc actuator and is capable of being set to an accurately defined operating temperature, but has also been found to be susceptible to stress cracking.
The actuator of British Patent Specification No. 1,542,252 was designed particularly to overcome the stress cracking problems of the prior art, and in its preferred embodiment comprises a dished circular member of sheet bimetal having a curved aperture therein which, similarly to the actuator of British Patent Specification No. 657,434, releases a tongue from the bimetal member. The aperture has an outer periphery in the form of an arc of a circle which merges smoothly with the inner periphery of the aperture at rounded ends adjacent the root of the released tongue, this configuration having been designed to minimize stresses in the actuator when it snaps between its oppositely dished configurations. Reference may be made to British Patent Specification No. 1,542,252 for a full discussion of the problems associated with the prior art actuators and of the design considerations involved in the new actuator of British Patent Specification No. 1,542,252.
While the improved actuator of British Patent Specification No. 1,542,252 does have some advantageous features, it is unfortunately wasteful in material, and rectangular blade actuators of the general form described in British Patent Specification No. 657,434 continue to be widely used, particularly though not exclusively with the bimetal blade mounted by its center leg (i.e., its released tongue). It has been found that when such rectangular blade actuators are used as overload current sensing elements in circuit breakers, where the blade carries the current to be monitored and is responsive to the heating effect of such current flow, several problems are experienced.
The heating pattern in the bimetal blade is uneven and both ends of the blade tend to run cool, which gives rise to complex stresses in the bimetal. Furthermore, the end of the blade nearest the free or tip end of the released tongue, i.e., the end which would conventionally have taken a crimp for imparting the dished configuration to the blade, is quite broad and can itself snap at a different temperature from the other end of the blade nearest the root end of the released tongue, which is the end one would normally consider as constituting the snap action part of the bimetal; this effect is particularly marked at low temperatures (e.g., -40.degree. C.) and, with the bimetal mounted by its central leg, has resulted in the larger end of the bimetal nearest the root end of the tongue snapping over center and the contacts of the switch nonetheless staying closed until the other (contact) end of the bimetal has snapped over at a different temperature.